1. Field of the Invention
The present invention relates to a circuit structure, and more particularly, to a circuit structure which effectively avoids the crosstalk between signals.
2. Description of the Related Art
In a large size PCB (Printed Circuit Board) and package substrate the width of the signal lines for electrically coupling two components or two terminals must be fixed to maintain a constant characteristic impedance of the signal lines when the electronic signal is transmitted in the signal lines. In addition, in the general application and design since the distance between two signal lines is reduced, the capacitive coupling and the inductive coupling effect, occurred between the signals in the signal lines, is increased accordingly. Those coupling effect deteriorates near-end crosstalk and far-end crosstalk, thus signals cannot be accurately transmitted from one terminal to the other terminal of the signal line. Especially, when transmitting a signal in high-speed and high frequency, an unexpected noise crosstalk is occurred in another signal line due to the interference of the electromagnetic field.
FIGS. 1A and 1B schematically show a top view and a sectional view along with the line I—I in FIG. 1A of a conventional circuit structure, respectively. Herein, a general circuit substrate 100 is exemplified. A plurality of lines 110, 112, 114, 116 with same line width are disposed on an upper surface 122 of a dielectric layer 120, and a lower surface 124 of the dielectric layer 120 is disposed on a reference plane 130. Wherein, the distances between the respective lines 110, 112, 114, 116 and the reference plane 130 are all the same. The reference plane 130 is, for example, a power plane or a ground plane.
Note that in order to avoid crosstalk occurred on another signal, which influences the signal transmission quality, due to an electromagnetic field coupling occurred between the line 110 and the neighboring line 112 during transmitting a signal through the line 110, it is common in the prior art to increase the distances D1, D2 between the lines 112, 114 and another lines 110, 116. However, with such method, the free layout space of the substrate 100 is reduced inevitably.